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Zero Crossing Detectors and Comparators(The Unsung Heros of Modern Electronics Design)

Rod Elliott (ESP)

Introduction

Zero crossing detectors as a group are not a well-understood application, although they are essentialelements in a wide range of products. It has probably escaped the notice of readers who have looked

at the lighting controller and the Linkwitz Cosine Burst Generator, but both of these rely on a zero

crossing detector for their operation.

A zero crossing detector literally detects the transition of a signal waveform from positive and

negative, ideally providing a narrow pulse that coincides exactly with the zero voltage condition. At

first glance, this would appear to be an easy enough task, but in fact it is quite complex, especially

where high frequencies are involved. In this instance, even 1kHz starts to present a real challenge if

extreme accuracy is needed.

The not so humble comparator plays a vital role - without it, most precision zero crossing detectors

would not work, and we'd be without digital audio, PWM and a multitude of other applicationstaken for granted.

Basic Low Frequency Circuit

Figure 1 shows the zero crossing detector as used for the dimmer ramp generator in Project 62. This

circuit has been around (almost) forever, and it does work reasonably well. Although it has almost

zero phase inaccuracy, that is largely because the pulse is so broad that any inaccuracy is completely

swamped. The comparator function is handled by transistor Q1 - very basic, but adequate for the

job.

The circuit is also sensitive to level, and for acceptable performance the AC waveform needs to beof reasonably high amplitude. 12-15V AC is typical. If the voltage is too low, the pulse width will

increase. The arrangement shown actually gives better performance than the version shown in

Project 62 and elsewhere on the Net. In case you were wondering, R1 is there to ensure that the

voltage falls to zero - stray capacitance is sufficient to stop the circuit from working without it.

Figure 1 - Basic 50/60Hz Zero Crossing Detector

The pulse width of this circuit (at 50Hz) is typically around 600us (0.6ms) which sounds fast

enough. The problem is that at 50Hz each half cycle takes only 10ms (8.33ms at 60Hz), so the pulse

width is over 5% of the total period. This is why most dimmers can only claim a range of 10%-90%- the zero crossing pulse lasts too long to allow more range.

While this is not a problem with the average dimmer, it is not acceptable for precision applications.

For a tone burst generator (either the cosine burst or a 'conventional' tone burst generator), any

inaccuracy will cause the switched waveform to contain glitches. The seriousness of this depends

on the application.

Precision zero crossing detectors come in a fairly wide range of topologies, some interesting, others

not. One of the most common is shown in Project 58, and is commonly used for this application.

The exclusive OR (or XOR) gate makes an excellent edge detector, as shown in Figure 2.

Figure 2 - Exclusive OR Gate Edge Detector

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There is no doubt that the circuit shown above is more than capable of excellent results up to quite

respectable frequencies. The upper frequency is limited only by the speed of the device used, and

with a 74HC86 it has a propagation delay of only 11ns [1], so operation at 100kHz or above is

achievable.

The XOR gate is a special case in logic. It will output a 1 only when the inputs are different (i.e. one

input must be at logic high (1) and the other at logic low (0v). The resistor and cap form a delay so

that when an edge is presented (either rising or falling), the delayed input holds its previous valuefor a short time. In the example shown, the pulse width is 50ns. The signal is delayed by the

propagation time of the device itself (around 11ns), so a small phase error has been introduced. The

rise and fall time of the squarewave signal applied was 50ns, and this adds some more phase shift.

There is a pattern emerging in this article - the biggest limitation is speed, even for relatively slow

signals. While digital logic can operate at very high speeds, we have well reached the point where

the signals can no longer be referred to as '1' and '0' - digital signals are back into the analogue

domain, specifically RF technology.

The next challenge we face is converting the input waveform (we will assume a sinewave) into

sharply defined edges so the XOR can work its magic. Another terribly under-rated building block

is the comparator. While opamps can be used for low speed operation (and depending on the

application), extreme speed is needed for accurate digitisation of an analogue signal. It may not

appear so at first glance, but a zero crossing detector is a special purpose analogue to digital

converter (ADC).

Comparators

The comparator used for a high speed zero crossing detector, a PWM converter or conventional

ADC is critical. Low propagation delay and extremely fast operation are not only desirable, they are

essential.

Comparators may be the most underrated and under utilised monolithic linear component.

This is unfortunate because comparators are one of the most flexible and universally

applicable components available. In large measure the lack of recognition is due to the IC

opamp, whose versatility allows it to dominate the analog design world. Comparators are

frequently perceived as devices that crudely express analog signals in digital form - a 1-bit

A/D converter. Strictly speaking, this viewpoint is correct. It is also wastefully constrictive

in its outlook. Comparators don't just compare in the same way that opamps don't "just

amplify". [2]

The above quote was so perfect that I just had to include it. Comparators are indeed underrated as a

building block, and they have two chief requirements ... low input offset and speed. For the

application at hand (a zero crossing detector), both of these factors will determine the final accuracyof the circuit. The XOR has been demonstrated to give a precise and repeatable pulse, but its

accuracy depends upon the exact time it 'sees' the transition of the AC waveform across zero. This

task belongs to the comparator.

Figure 3 - Comparator Zero Crossing Detector

In Figure 3 we see a typical comparator used for this application. The output is a square wave,

which is then sent to a circuit such as that in Figure 2. This will create a single pulse for each

squarewave transition, and this equates to the zero crossings of the input signal. It is assumed for

this application that the input waveform is referenced to zero volts, so swings equally above and

below zero.

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Figure 4 - Comparator Timing Error

Figure 4 shows how the comparator can mess with our signal, causing the transition to be displaced

in time, thereby causing an error. The significance of the error depends entirely on our expectations

- there is no point trying to get an error of less than 10ns for a dimmer, for example.

The LM339 comparator that was used for the simulation is a very basic type indeed, and with a

quoted response time of 300ns it is much too slow to be usable in this application. This is made a

great deal worse by the propagation delay, which (as simulated) is 1.5us. In general, the lower the

power dissipation of a comparator, the slower it will be, although modern IC techniques have

overcome this to some extent.

You can see that the zero crossing of the sinewave (shown in green) occurs well before the output

(red) transition - the cursor positions are set for the exact zero crossing of each signal. The output

transition starts as the input passes through zero, but because of device delays, the output transition

is almost 5us later. Most of this delay is caused by the rather leisurely pace at which the output

changes - in this case, about 5us for the total 7V peak to peak swing. That gives us a slew rate of

1.4V/us which is useless for anything above 100Hz or so.

One of the critical factors with the comparator is its supply voltage. Ideally, this should be as low as

possible, typically with no more than 5V. The higher the supply voltage, the further the output

voltage has to swing to get from maximum negative to maximum positive and vice versa. While a

slew rate of 100V/us may seem high, that is much too slow for an accurate ADC, pulse width

modulator or zero crossing detector.

At 100V/us and a total supply voltage of 10V (5V), it will take 0.1us (100ns) for the output to

swing from one extreme to the other. To get that into the realm of what we need, the slew rate

would need to be 1kV/us, giving a 10ns transition time. Working from Figure 3, you can see that

even then there is an additional timing error of 5ns - not large, and in reality probably as good as wecan expect.

The problem is that the output doesn't even start to change until the input voltage passes through the

reference point (usually ground). If there is any delay caused by slew rate limiting, by the time the

output voltage passes through zero volts, it is already many nanoseconds late. Extremely high slew

rates are possible, and Reference 2 has details of a comparator that is faster than a TTL inverter!

Very careful board layout and attention to bypassing is essential at such speeds, or the performance

will be worse than woeful.